Celsus is often cited as a Pagan author who was supposedly a "Pagan Monotheist". However, if one actually reads Celsus, one finds that this is not true, as the following shows.

"... without rational cause the goatherds and shepherds followed Moses, who taught them that there was but one God - deluded, apparently, by his rather naive beliefs - and caused them to forsake their natural inclinations to credit the existence of many Gods. For our part, we acknowledge the many: Mnemosyne, who gave birth to the Muses by Zeus; Themis, Mother of the Hours; and so on. Yet those goatherds and shepherds came to believe in one God and called him the Most High - Adonai, the Heavenly One - or sometimes Sabaoth, or whatsoever - and came to discredit all other Gods....

"The Christians ignore the good offices of the Dioscori, of Herakles, Asclepios and of Dionysus, and say that these are not Gods because they were humans in the first place. Yet they profess belief in a phantom god who appeared only to members of his little club, and then, so it seems, merely as a kind of ghost."

Taken from Hoffman's edition of "Celsus on the True Doctrine".

Also, Epicurus, who is often portrayed as some kind of atheist, observed: "For there are Gods -- the knowledge of them is self-evident" Epicurus, Letter to Menoeceus (Long & Sedley "Hellenistic Philosophers" vol. 1, p. 140)